Memory devices for storing digital data are abundant in today's computers, automobiles, cell telephones and media information cards. Certain of these memory devices or storage elements, referred to as non-volatile memory, retain the stored digital data when power is removed from the device. For example non-volatile memory instructions instruct a computer during the boot-up process and store instructions and data for sending and receiving calls in a cellular telephone. Electronic products of all types, from microwave ovens to heavy industrial machinery, store their operating instructions in these non-volatile storage elements. Certain non-volatile memory devices offer multiple programming capabilities, with previously stored information overwritten by new data. Other non-volatile devices provide only one-time programmability.
Another class of memory devices, volatile memory devices, looses the stored information when power is removed. Dynamic random access memories (DRAM) and static random access memories (SRAM) are two types of volatile storage elements.
A read-only memory (ROM) is one type of permanent data storage non-volatile memory. Once stored in the ROM device, the data cannot be overwritten or otherwise altered. The ROM is “programmed” during its manufacture by making permanent electrical connections in selected memory cells. Since the ROM is programmed during the design stage, the stored information can be changed only by redesigning the ROM integrated circuit.
A programmable read-only memory (PROM) is a non-volatile device that is programmable after fabrication, but is programmable only once. In one type of PROM, each memory cell comprises a fusible link. The PROM is “programmed” by opening or blowing a fusible link in selected cells, while other links remain intact. A PROM can be programmed during or after fabrication by the manufacturer, or later by a purchaser. Advantageously, manufacturers can offer a single PROM hardware design that is user-programmable. Typically, the PROM includes one or more external pins for receiving current from an external source to open the fusible links.
An erasable programmable read-only memory (EPROM) is another non-volatile memory device, but an EPROM can be erased and reprogrammed as desired. The EPROM is programmed electronically and erased using ultraviolet light passing through an ultraviolet-permeable quartz window formed in the package. An EEPROM (electronically erasable programmable read-only memory) is yet another type of read-only memory that can be programmed, electronically erased and electronically reprogrammed.
A flash EEPROM memory is a type of EEPROM non-volatile memory that is especially prevalent in electronic devices where the user desires to add or change information after the memory device has been fabricated and inserted into the electronic device. For example, flash memory allows the user to add addresses and calendar entries in a personal digital assistant and erase and re-use media cards that store pictures taken with a digital camera. Flash memory devices differ form other EEPROM devices in that a flash memory permits entire banks or a large number of stored data words to be simultaneously erased, whereas other EEPROM devices permit the simultaneous erasure of only single words. Thus erasing a large memory block in a non-flash EEPROM is a much slower process than the same operation in a flash memory. Also, a flash EEPROM is typically smaller than other types of EEPROM memory devices.
An anti-fuse (comprising gallium oxide or amorphous silicon, for example) is another PROM non-volatile memory device. The anti-fuse is formed in an open state and can be programmed to a closed state using a voltage that is higher than the normal operating supply voltage for integrated circuits. Therefore, transistors in the programming circuitry of the anti-fuse device must be fabricated with higher junction break down voltages than the conventional transistor. Further, as newer integrated circuit process technologies employ reduced gate oxide thicknesses the fabricated devices require higher well doping levels, which results in even lower junction break down voltages. Thus the anti-fuse devices are becoming less compatible with advancing process technologies. Also, certain of the anti-fuse materials are not compatible with standard CMOS fabrication processes.
Certain non-volatile memory devices are referred to as “one-time programmable (OTP),” memories, including anti-fuse devices, EPROM's and PROM's. OTP memory can be further subdivided into those with relatively large arrays of storage elements (cells) and those with a relatively small number of cells. OTP devices with few cells are useful for trimming analog circuit device parameters (e.g., fuses are placed to short out or insert resistors within a serial string of resistors, thereby adjusting the total string resistance) and for permanently storing a relatively small number of non-modifiable data bits, such as for providing external identification of an integrated circuit chip by reading stored identification bits with an off-chip reader.
Another type of OTP non-volatile memory comprises conductive fuse storage elements disposed in an interconnect layer of an integrated circuit. Depending upon the process technology selected, a material of the conductive layer comprises polysilicon, metal or a silicide. Certain of these OTP devices comprise fuses formed on an upper layer interconnect structure. Other devices comprise buried fuses formed in lower level interconnect structures. Whether formed in the upper or lower level interconnect structures, the fuse is formed coincident with the formation of the interconnect structure by adding fuse features to the interconnect structure mask.
One technique uses a laser for programming (i.e., blowing) conductive fuse storage elements on the top layer of the interconnect structure. The integrated circuit is masked to expose the fuses to be opened, and laser energy is directed at selected exposed fuses to open them.
FIG. 1 illustrates another prior art circuit for blowing one or both fuses 10 and 12. The fuse 10 is connected between a source/drain 15 of a MOSFET 16 and ground. The fuse 12 is connected between a source/drain 17 of a MOSFET 18 and ground. A second source/drain 19 and 20 of the MOSFETs 16 and 18, respectively, are connected to a voltage or current source. To blow the fuse 10, a voltage Vg1 is applied to a gate 21 of the MOSFET 16, turning on the MOSFET 16 and permitting large current flow from the voltage or current source through the source/drain 19, the MOSFET channel and the source/drain 15 through the fuse 10, opening the fuse material. The fuse 12 is blown in a similar manner, through the MOSFET 18 by the application of a turn-on voltage to a gate 22 of the MOSFET 18. The current required to blow the fuses 10 and/or 12, typically about 10 to 50 mA, requires the use of relatively large MOSFET's. These transistors consume an area on the order of 1,000 microns2 with a gate width of about 500 microns. Because of their large size, the MOSFETS 16 and 18 are more expensive to fabricate than smaller MOSFETs fabricated in most of today's integrated circuit devices and also consume valuable area on the integrated circuit.